In vtkPVPostFilter, replace input type whitelist with vtkDataObject.

Originally, the accepted inputs to vtkPVPostFilter were a whitelist of concrete data types in VTK (i.e. no abstract types). This was done to prevent vtkTemporalDataSet from being accepted by the filter. However, it also prevented user-defined datatypes from passing through the filter. Since vtkTemporalDataSet has been removed, this safeguard is no longer necessary. So, vtkPVPostFilter now accepts vtkDataObject and all of its derived types.